Modular multi-cell batteries represent a total departure from traditional designs for SLI automotive lead-acid batteries. Traditional designs incorporate a premolded container divided into individual cell compartments. Battery plates and separators are separately formed and assembled and then are inserted into the cell compartments during assembly of the battery.
Modular battery designs, such as those disclosed in U.S. patent application Ser. No. 762,814, filed Sep. 17, 1991 (to be issued on Oct. 26, 1993 as U.S. Pat. No. 5,256,502), U.S. Pat. No. 5,068,160 to T. Clough et al., U.S. Pat. Nos. 4,239,839 and 4,209,579 to W. McDowell and U.S. Pat. Nos. 4,022,951 and 3,941,615 to W. McDowell, however, do not have a premolded container. Instead, the container, such as it is, is formed by joining together a collection of thermoplastic frames.
More specifically, modular lead-acid batteries are fabricated from a plurality of relatively thin, generally rectangular, prefabricated thermoplastic frames. Some frames support electrochemically active battery plates and, in some designs, other frames support separator material. During assembly of the battery, the frames are stacked and joined together to create an assembly of alternating plate frames and separator frames sandwiched between two end frames. In their joined state, the collection of frames constitutes an electrolyte-tight container.
In some designs, the frames include a number of partition ribs which extend across the peripheral members of the frames. When a battery is assembled from the frames, the peripheral members of the frames are joined together to form the battery container. The partition ribs are joined together to form dividers which define a plurality of cells between adjacent plate-supporting frames.
It is possible to join an entire stack of frames together at the same time in a single induction or conduction heat-sealing operation. Such methods, however, require that the frames incorporate a metallic conductor or inductor near the sealing area of the frame, a feature which makes the frame much more expensive. As a practical matter, therefore, modular frames having partition ribs and the like which will form cell dividers in a finished battery must be joined together sequentially. That is, two frames must be sealed together, a third sealed to the first two, a fourth sealed to the first three, and so on.
There are a number of general techniques for sequentially joining modular battery frames, such as heat-sealing with, for example, a hot-plate or focused infra-red radiation. A relatively large amount of plastic is melted, however, in heat-sealing processes. Thus, there is a relatively long cooling period during which pieces must be allowed to set, and heat-sealing modular battery frames sequentially is relatively slow.
The frames also may be joined together with solvents or adhesives, such as epoxy, hot-melt and other electrolyte-insensitive solvents and adhesives traditionally used in manufacturing batteries. Modular batteries, however, require extensive sealing. For example, a modular battery might incorporate from about 10 to about 70 or more frames. Laying down beads of solvent or adhesive to form all the necessary seals is relatively slow, and the adhesive or solvent itself usually is relatively expensive and harmful to the environment.
Ultrasonic welding is another option. Given the size of the typical modular frame, however, ultrasonic horns used to perform such operations must be larger and have higher energies, and horns of sufficient size and energy are not durable enough for use in mass assembly of batteries.
Yet another method involves vibrationally welding the frames together. In such methods, for example, two frames are placed next to each other and lightly pressed together. One of the frames is fixed and the other is mechanically vibrated linearly or orbitally in the plane in which the frames contact each other. The vibration causes plastic at the interface between the frames to melt and, upon cooling, to form a weld between the frames. Thereafter, additional frames are vibration welded in turn to the accumulating stack of welded frames until the frame assembly is completed.
Vibrational welding in general can reliably and efficiently join frames into a leak-tight container having a number of dividers. There is a very short cooling period, and thus, vibrational welding is faster than sequential heat sealing methods. Vibrational welding also is faster and cheaper than using adhesives. Accordingly, vibrational welding has been used successfully in the manufacture of flooded modular batteries.
Applicants have observed, however, that recombinant modular batteries assembled by vibrational welding methods exhibit short circuiting. That short circuiting is believed to be caused by damage to the separator which occurs during vibration welding. That is, the electrochemistry of oxygen-recombinant, valve-regulated batteries necessitates certain modification of the separator as compared to the separators in flooded batteries. In flooded batteries, the separator is relatively thin, and ideally it does not contact the surface of the active plates.
In recombinant batteries, however, there must be firm contact between a separator and the surface of a battery plate, as this contact aids in transporting oxygen from the positive plate to the negative plate for recombination and ensures efficient electrochemical communication between the plates and electrolyte.
In recombinant batteries having a traditional design, intimate contact is achieved by oversizing the separator somewhat relative to the distance which will separate the plates in a finished battery. During assembly, the oversized separators are disposed between a series of plates, and the plates are squeezed together, welded to a strap, and placed in the battery container, such that the separator is compressed in the finished battery. The compressed, oversized separator provides a certain amount of pressure which forces the separator into intimate contact with the surface of the plates.
In attempting to use vibrational welding methods to assemble recombinant modular batteries, however, applicants have observed that the contact between a oversized separator and plate can result in severe damage to the separator material and embedding of electrochemically active paste in the separator. This in turn, can lead to short circuiting of the cell.
An object of the subject invention, therefore, is to provide a method of assembling a modular lead-acid battery which is reliable and economical and which is faster than sequentially bonding frames together by heat sealing or with adhesives. A related and more specific object is to provide a method of assembling modular batteries, including recombinant modular batteries having cell dividers, by vibrationally welding frames together without damaging the separator.
Those and other objects and advantages of the invention will be apparent to those skilled in the art upon reading the following detailed description and upon reference to the drawings.